Damages on the insulation of roof systems has consequences for the performance of the roof system and the energy cost. An early detection of roof thermal anomalies may greatly reduce long-term maintenance costs. Airborne thermographic methods have for several years with success been applied to detect thermal anomalies of roof systems. For ventilated attics, the surface temperature pattern of the attic insulation can not always be inferred from a measurement of the surface temperature due to the influence of thermal properties of the roof system and to weather conditions. In this paper is presented a calculation of how the surface temperature pattern of the insulation influences the surface temperature pattern of a flat roof when part of the insulation is not functioning properly due to, for example, wettening of the material. The results are presented for various attic geometries, amount of convective heat transfer from the insulation to the attic air for different wind speeds, for various internal-external temperature differences, and for different emissivities of the roofing material.